The operation of semiconductor devices such as laser diodes and photodiodes is sensitive to the device temperature. For example, the laser emission wavelength will typically shift as the device temperature rises. This shift is undesirable and, for optimal results, should be avoided. In many applications the laser diode is mounted on a separate substrate than that of the electronics that drives the laser. Physical separation is required so that the power dissipated by the laser driver circuit does not heat the laser diode. Physical separation, however, complicates system integration and packaging. For example, since separate substrates are used for mounting the laser and the laser driver chip, long wire traces are required to electrically connect the two together. The design of such wire (typically referred to as transmission lines at the frequency of operation) is not trivial. Moreover, to reduce losses the transmission lines should be made on insulating substrates which further limit the choice of substrate on which the laser diode may be mounted. Integrating the driver circuit and the laser diode on the same substrate is desirable since it would eliminate excess transmission lines and an insulating substrate. Because silicon is a good heat conductor, the close integration of the laser driver and the laser diode on the same silicon substrate is not possible for the reasons explained above.
It is therefore desirable to provide a substrate structure which enables thermal isolation between at least two regions of the substrate. The isolation structure should occupy a small area on the substrate. In addition to thermal isolation, efficient cooling of hot regions is also needed to reduce thermal cross-talk between the two regions.
Integration of digital components with analog or RF components on a single chip for System on a Chip (“SOC”) applications or on a single substrate for System on a Package applications is highly desirable to improve system performance and to achieve higher level of miniaturization. Digital components tend to be noisy and disturb the performance of sensitive low-noise RF or analog components. For an effective system, it is desirable to electrically shield the RF/analog components from the digital logic components.